Gas turbines are widely used in commercial operations for power generation. A typical gas turbine includes a compressor at the front, one or more combustors around the middle, and a turbine at the rear. The compressor and the turbine typically share a common rotor.
The compressor includes multiple stages of compressor blades attached to the rotor. Ambient air enters an inlet of the compressor, and rotation of the compressor blades imparts kinetic energy to the working fluid (air) to bring it to a highly energized state. The compressed working fluid exits the compressor and flows to the combustors where it mixes with fuel in the combustors. The mixture of the compressed working fluid and fuel ignites in the combustors to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases exit the combustors and flow to the turbine where they expand to produce work.
The turbine includes alternating rows of rotating turbine blades or turbine buckets and stationary nozzles or stators enclosed in a casing. As the combustion gases from the combustors pass over the turbine buckets, the combustion gases expand, causing the turbine buckets to rotate. The combustion gases then flow to the stators which redirect the combustion gases to the next row of rotating turbine buckets, and the process repeats for the following stages.
The thermodynamic efficiency of the gas turbine may be increased by operating the gas turbine at higher temperatures. For example, higher temperature combustion gases contain more energy which produce more work as the combustion gases expand across the turbine buckets. Increased temperatures, however, have a detrimental affect on the strength of the turbine components. For example, nickel or cobalt alloys, such as Inconel 617, Haynes 188, and Haynes 230, are commonly used in turbine buckets because of their ductility, ease in welding, and long fatigue life. However, the strength of these nickel and cobalt alloys decreases as the temperature increases. The reduced strength at higher temperatures produces swelling or creep in the turbine components, particularly at the tip of the turbine buckets, which may result in an unacceptable clearance between the rotating turbine buckets and the casing. As a result, turbine buckets made from nickel or cobalt alloys typically require reduced combustion temperatures, additional cooling systems to limit the maximum temperature of the turbine buckets, and/or increased maintenance and inspection cycles.
A variety of techniques are used to allow turbines to operate with higher temperature combustion gases. For example, working fluid may be extracted from the compressor and supplied to the turbine to cool the higher temperature stages in the turbine. However, the use of working fluid to cool the turbine reduces the overall thermodynamic efficiency of the gas turbine. Additional manufacturing techniques, such as directional solidification and improved heat treatments, may be utilized to manufacture turbine components to allow the turbine to operate at higher temperatures. These additional manufacturing techniques, however, increase the time and cost to manufacture the turbine components.
Another technique to allow turbines to operate at higher temperatures is to incorporate new materials, specifically precipitation hardened superalloys into the design of the turbine components. These superalloys have improved strength at higher temperatures, reducing the onset of swelling or creep at operating temperatures during the life of the components. However, the high strength precipitation hardened alloys have lower ductility and are in general difficult to weld.
Therefore, the need exists for improved turbine components that can operate at increasingly higher temperatures. In addition, the need exists for improved manufacturing methods that may improve the integrity of weld joints between the turbine components.